I've been following
GameFAQs
even though all the guides are PS2 specific, the game is a fairly straight port, so the 'world' is the same, just controls are different.
of particular use is the guide by
dark52
and the
100% Guide
I've been using these 2 documents (and some that they mention in their text) and I've done about 30% of the game in the last 2 weeks with very casual play 'cause I've got a job and a life, both of which interfere with GTA:VC, or the other way around....
enjoy!